Effect of A-site cation on crystal structure and microwave dielectric properties of AGe4O9 (A = Ba, Sr) ceramics

AGe 4 O 9 (A = Ba, Sr) ceramics with hexagonal structure were fabricated by solid-phase reaction. BaGe 4 O 9 sintered at 1060 °C had low ε r ∼9.8, high Q × f ∼78,400 GHz and τ f ∼ -44.2 ppm/°C, whilst SrGe 4 O 9 sintered at 1100 °C exhibited relatively high ε r ∼11.2, low Q × f ∼62,500 GHz and near-zero τ f ∼ -11.7 ppm/°C. The τ f of SrGe 4 O 9 was nearer to zero than that of BaGe 4 O 9 due to the smaller τ ε value and the smaller distortion of the [GeO 6 ] octahedrons. The ‘rattling’ cations at A-site contribute to comparatively high ε r , low Q × f and near-zero τ f values in Sr 1- x Ba x Ge 4 O 9 ceramics. This work presents a new idea of adjusting the microwave dielectric performance (MDP) of ceramics by changing the strength of the rattling effect at A-site.